The present invention relates to monitoring X-ray tubes, and more particularly, to monitoring position, intensity, and focussing of the emitting spot of a microfocus X-ray tube.
When examining industrial objects, e.g., composite structures for aircraft engines, etc., for flaws, it is desirable to be able to detect even very small flaws. For this reason, microfocus X-ray tubes, which have a means for focussing the electron beam impinging on the anode, are used as the radiation source for high resolution radiography of such objects. These tubes can produce sharp images of small flaws or features because they approximate a point source of X-rays. In particular, their X-ray emitting spot has a diameter of about 20 to 50 .mu.m compared with a diameter of about 1 to 2 mm for a non-microfocus X-ray tube. The X-rays that penetrate the object are usually passed through a collimator in order to reject scattered X-rays and help define the inspected region. The collimated X-rays are then detected and the detected signal is usually applied to a computer so that tomography can be performed.
A constant intensity signal is required for computer tomography. In order to achieve this, the anode-cathode voltage difference of the tube is regulated so that constant energy X-rays are emitted, and thus the penetration of the X-rays into the object is a constant. Further, the anode current is sensed and applied to a control grid voltage determining circuit in order to keep said current, and thus the amount of the X-rays, a constant. However, high quality imaging also requires careful control of the position and size of the X-ray emitting spot on the anode. This has been done by viewing the displayed image of a system having a collimator. Then the focussing means is adjusted for greatest image intensity since in systems having a collimator, the focussing means adjustment providing the greatest image intensity also provides the sharpest image. Alternatively, for systems not having a collimator, a fluoroscope is used to obtain a displayed image, and then the focussing means is adjusted for sharpest image. Unfortunately, the first of these processes is not "real time" in that the apparatus cannot be imaging the indus-trial object when this process is performed since the data obtained will be invalid. This allows spot defocussing, caused by changes in tube geometry due to thermal deformation, to occur during imaging of the object. The second process is bulky and expensive.
It is therefore an object of the present invention to provide a "real time", compact, and inexpensive focussing and intensity adjustment apparatus and method for a microfocus X-ray tube.
It is another object to provide a microfocus X-ray tube for use in such an apparatus and method.